Eddie Sez:

Losing your ability to keep the airplane precisely on altitude is becoming a bigger deal every day, as the skies are becoming more tightly packed. Because each situation is likely to be unique, there are no cut and dried rules that always apply.

Appendix 5 of AC 91-85 contains information on procedures that are unique to oceanic RVSM airspace where direct voice communications between pilots and air traffic control (ATC) is not available and provides several scenarios as examples.

You should also refer to: International Abnormal Procedures / Special Procedures for In-flight Contingencies in Oceanic Airspace.

What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.

ICAO Contingency Procedures

[ICAO Doc 9574, ¶5.1.1 h)] the following contingency procedures should be adhered to after entering RVSM airspace:

  1. the pilot should notify ATC of contingencies (equipment failures, weather conditions) in which the ability to maintain CFL is affected and coordinate a plan of action;

  2. equipment failures should be notified to ATC. Some examples are:
    • failure of all automatic altitude-keeping devices on board the aircraft;

    • loss of redundancy of altimetry systems, or any part of these, on board the aircraft;

    • failure of all altitude-reporting transponders;

    • loss of thrust on an engine necessitating descent; and

    • any other equipment failure affecting the ability to maintain CFL;
  3. the pilot should notify ATC when encountering severe turbulence; and

  4. if unable to notify ATC and obtain an ATC clearance prior to deviating from the assigned CFL, the pilot should follow established contingency procedures as defined by the region of operation and obtain ATC clearance as soon as possible.

North Atlantic Procedures

Contingencies Within MNPS Airspace

[ICAO Nat Doc 001, ¶3.10]

Contingencies Within RVSM Airspace

[ICAO Nat Doc 001, ¶3.11]

Other Regional Differences

The basic general procedures are outlined below. Consult AIPs, Jeppesen State Pages, and ICAO Document 7030, Regional Supplementary Procedures, for regional differences. [AC 91-85, Appendix 5]

Example Scenarios

Appendix 5 of AC 91-85 Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum Airspace provides several scenarios to summarize pilot actions to mitigate the potential for conflict with other aircraft in certain contingency situations.

Scenario 1 — "Unsure"

[AC 91-85, Appendix 5, ¶5.b.(1)]

The pilot is: 1) unsure of the vertical position of the aircraft due to the loss or degradation of all primary altimetry systems, or 2) unsure of the capability to maintain CFL due to turbulence or loss of all automatic altitude control systems.

Scenario 2 — Bad Altimeter

[AC 91-85, Appendix 5, ¶5.b.(2)]

There is a failure or loss of accuracy of one primary altimetry system (e.g., greater than 200 ft difference between primary altimeters).

The Pilot should cross check standby altimeter, confirm the accuracy of a primary altimeter system and notify ATC of the loss of redundancy. If unable to confirm primary altimeter system accuracy, follow pilot actions listed in the preceding scenario.

Scenario 3 — All Altitude Control Systems Fail

[AC 91-85, Appendix 5, ¶6.a.(1)]

If all automatic altitude control systems fail, the pilot should:

Scenario 4 — Loss of Redundancy in Primary Altimetry Systems

[AC 91-85, Appendix 5, ¶6.b.(2)]

There is a loss of accuracy in primary altimetry systems.

If the remaining altimetry system is functioning normally, the pilot should couple that system to the automatic altitude control system, notify ATC of the loss of redundancy and maintain vigilance of altitude keeping.

Scenario 5 — All Primary Altimetry Systems are Considered Unreliable or Fail

[AC 91-85, Appendix 5, ¶6.b.(3)]

If all primary altimetry systems fail, the pilot should:

Scenario 6 — Divergence

[AC 91-85, Appendix 5, ¶6.d.]

If the primary altimeters diverge by more than 200 ft (60 m), the pilot should:

Scenario 7 — Turbulence

[AC 91-85, Appendix 5, ¶6.e.]

In the event of turbulence (greater than moderate) which the pilot believes will impact the aircraft’s capability to maintain FL, the pilot should:

Book Notes

Portions of this page can be found in the book International Flight Operations, Part VI, Chapter 4.


Advisory Circular 91-85, Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum Airspace, 8/21/09, U.S. Department of Transportation

ICAO Doc 4444, Procedures for Air Navigations Services, Amendment 2

ICAO Doc 7030 - Regional Supplementary Procedures, International Civil Aviation Organization, 2008

ICAO Doc 9574 - Manual on Implementation of a 300 m (1,000 ft) Vertical Separation Minimum Between FL 290 and FL 410 Inclusive, Second Edition, International Civil Aviation Organization, 2001

ICAO NAT Doc 001, Guidance and Information Material Concerning Air Navigation in the North Atlantic Region, Seventh Edition, January 2002

ICAO NAT Doc 007, North Atlantic Operations and Airspace Manual, Edition 2014/2015